The present invention relates generally to color image pickup apparatus such as color video camera, and more particularly to a color stripe filter which is disposed in a light path directing to a photo-electric conversion section of a color image pickup tube for separation of object-imaging light into three color signals which are in turn derived as a color multiplexed signal by the color image pickup tube.
Light from an object viewed from a color image pickup apparatus is focused on a photo-electric conversion target through a color stripe filter so as to be converted to an electrostatic image which is scanned in raster form by an electron beam under control of a deflection system to thereby produce a color multiplexed signal corresponding to the light image of the object. The color stripe filter generally comprises a plurality of recurrent groups of color stripes of different colors successively arranged at periodic intervals so that the color multiplexed signal includes a carrier signal component having a frequency, which is in inverse relation to the periodic interval of the recurrent groups of the color stripes, and modulated in amplitude and phase in accordance with color information. Here, when imaging an achromatic color object by the color image pickup apparatus, it should be required that the color multiplexed signal to be outputted from the image pickup tube includes only a luminance signal without containing color signals. One possible solution is that the peaks of transmission factors of the different color stripes is arranged to be equal to each other. However, the reproduced achromatic color signal tends to have a characteristic different from the relative luminosity factor, or spectral luminous efficiency, of the human eye. On the other hand, in cases where the reproduced achromatic color signal is arranged so as to have the characteristic substantially coincident with the spectral luminous efficiency, for example, in accordance with the known NTSC system, difficulty is generally encountered to meet the requirement in which the outputted color multiplexed signal contains only the luminance signal. Thus, a further improvement would be required from the viewpoint of meeting both the requirements at the same time.
Furthermore, in such a color image pickup apparatus using a color separation stripe filter, while the red color component and green color component can directly be obtained by the transmission lights of a red color stripe and a green color stripe of the color separation stripe filter, the blue color component is obtained by subtracting the green color component obtained by the green color stripe from the cyan color component obtained by a cyan color stripe thereof. In the case of surely obtaining the blue color component, it should be required that in the green region (500 to 600 nm in wavelength) the spectral characteristic of the cyan color stripe is similar in configuration to that of the green color stripe. Difficulty can be encountered to stably meet this requirement in conventional color separation stripe filters.